Matter of fact, FIL did have a will, leaving everything to be divided equally among his adult children.

The consensus among the siblings is that since SIL was taking care of FIL in his dotage, she earned whatever inheritance, if any, was left. So this is a case where she certainly didn't do the legal & financial things correctly, but since the affected parties aren't objecting, there shouldn't be any trouble.(Note to lurkers: this is the only case in the world where siblings are all in agreement. Get thee to an estate planning attorney, because your own siblings and children will NOT be in agreement!)

...SIL needs to consult a lawyer yesterday...

She's been consulting her lawyer son all along. Unfortunately, he's not an estate-planning attorney; and worse, he objected to the estate-planning attorney she did consult. So that's all a royal mess, and given that she's never been open to suggestions from DH & me, we just try to minimize our involvement. However, if she wants to send us a check, OK, we'll take that!